Wells for the production of petroleum products are drilled to a production zone with the well bore being lined with large diameter pipe, typically referred to in the industry as well casing. The well casing is perforated at the production zone to establish communication between the production zone and the well casing. During well completion operations, production tubing is placed in the well casing, there being one tubing string for each production zone intersected by the well bore. Packers are sometimes set in the casing to form a seal between the tubing and casing immediately above the level of the production zone although in pumping type wells packers are seldom used unless for control of specific problems such as water leaks in the casing, dual completions, etc. In some cases, the zone is produced by the pressure of natural gas contained within the production zone. In other cases, the production zone is produced by means of pumping or by means of gas lift operations.
Where the fluid produced from the production zone contains a substantial quantity of paraffin, the paraffin tends to become deposited on the inner surface of the tubing and in time will build up sufficient thickness to restrict the flow of fluid through the tubing. It is then necessary to remove the paraffin from the tubing in order that the well may be placed back into efficient production. One method for paraffin removal is through the use of hot liquid such as hot water or hot oil having a temperature that is elevated above the melting temperature of the paraffin. Hot oil is circulated through the tubing string to melt the paraffin and transport it to the surface where it is recovered. To accomplish circulation of liquid through the tubing string it is desirable to establish fluid communication between the tubing string and the annulus between the tubing string and casing and to inject the hot water or hot oil either into the casing or tubing. The hot liquid is then allowed to pass down the casing as tubing for a sufficient period to accomplish efficient removal of the melted paraffin.
As shown by U.S. Pat. No. 4,049,057, hot liquid may be injected into the annulus to induce flow of liquid from the annulus into the tubing. Thus, the liquid flows upwardly through the tubing to the surface and carries melted paraffin along with it. In this case, the well casing remains free from any contamination by the paraffin. Alternatively, the hot liquid may be injected into the tubing string to dissolve the paraffin and the hot oil and melted paraffin will then flow upwardly through the annulus between the tubing string and casing to the surface. A pressure differential circulating valve of this nature is set forth in U.S. Pat. No. 4,257,484. Similar valves function as balance valves as taught by U.S. Pat. No. 2,211,846. In some cases, the differential valves incorporate external sleeves as taught by U.S. Pat. Nos. 3,542,130 and 4,257,484 or internal sleeve valves as taught by U.S. Pat. No. 3,500,911. In most cases circulation valves are operated by differential pressure, but in some cases, as taught by U.S. Pat. No. 3,376,936, a sleeve valve may be operated from the surface by means of surface control equipment. Other U.S. patents of interest to this subject matter include U.S. Pat. Nos. 2,128,352; 2,488,649; 2,855,952; 3,542,130 and 3,750,749.
In view of the fact that pressure differential type circulating valves are often located beneath the surface of liquid standing in the annulus of the well, the valve must be designed for efficient operation even when subjected to hydrostatic pressure which might act to inhibit opening of the valve. It is also desirable that a pressure differential type circulating valve incorporate pressure responsive means to ensure that the valve open at a designed injection pressure and that it be caused to remain open at a much lower injection pressure, thus ensuring efficient circulation of liquid between the well casing and tubing.
Another typical problem with differential type circulation valves is the unusually high injection pressure that is required in order to accomplish valve opening and circulation. For example, it is not unusual for hot water or hot oil to be injected into a well for paraffin treatment at a pressure range of 2000 PSI to 2500 PSI or higher. Obviously, with liquid standing in the well, a portion of the tubing string and casing will be subjected to hydrostatic pressure of the liquid. The injection pressure during hot liquid treatment will simply be added to the hydrostatic pressure for determination of the total pressure to which the tubing string and casing are subjected. Obviously, pressure of this character can be detrimental to the service life of standing valves and it can also overstress the tubing and casing to such extent that repair or replacement will be nessssary within a reasonably short period of time. It is desirable, therefore, to provide for hot liquid treatment of paraffin coated tubing at injection pressures that are materially reduced in comparison to high pressure circulating valves.